A microbial fuel cell or biological fuel cell is a bio-electrochemical system that drives a current by mimicking bacterial interactions found in nature. Micro-organisms catabolize compounds such as glucose, acetate, butyrate or wastewater and can generate electrons with source streams carrying catabolizable compounds, including those identified above. The electrons gained from this oxidation are transferred to an anode, where they depart through an electrical circuit before reaching the cathode. Here they are transferred to a high potential electron acceptor such as oxygen. As current flows over a potential difference, power is generated directly from biofuel by the catalytic activity of bacteria.
The microbial fuel cell is a device that converts chemical energy to electrical energy by the catalytic reaction of microorganisms. A typical microbial fuel cell consists of anode and cathode modules separated by a cation specific membrane. In the anode module, fuel is oxidized by microorganisms, generating electrons and protons. In typical microbial fuel cells, electrons are transferred to the cathode module through an external electric circuit, and the protons are transferred to the cathode module through the membrane. Electrons and protons are consumed in the cathode module, combining with oxygen to form water.
Most of the microbial cells are electrochemically inactive. The electron transfer from microbial cells to the electrode is facilitated by mediators such as thionine, methyl viologen, methyl blue, humic acid, neutral red and so on. Most of the mediators available are expensive and toxic. Some microbial fuel cells do not require a mediator but use electrochemically active bacteria to transfer electrons to the electrode (electrons are carried directly from the bacterial respiratory enzyme to the electrode). Among the electrochemically active bacteria are, Shewanella putrefaciens, Aeromonas hydrophila, and others. Bacteria in such MFCs typically have electrochemically-active redox enzymes such as cytochromes on their outer membrane that can transfer electrons to external materials.
When micro-organisms consume nutrients in source water in aerobic conditions they produce carbon dioxide and water. However when oxygen is not present they produce carbon dioxide, protons and electrons. One reaction that may occur is as follows:C12H22O11+13H2O --->12CO2+48H++48e−In fact, all biodegradable materials, such as proteins, fats and lipids, among others, are broken down to reaction products in the degradation process. These materials are all potential sources of energy through the present invention.
In order to generate a useful current it is necessary to create a complete circuit, not just shuttle electrons to a single point.
Microbial fuel cells (“MFC”) have a number of potential uses. The first and most obvious is harvesting the electricity produced for a power source. Virtually any organic material could be used to ‘feed’ the fuel cell. Water used in various applications, such as food manufacturing and agriculture, create polluted waste streams. The organic matter from such streams can be used to feed the fuel cell. Manufacturers are required by regulation to clean such waste streams and current solutions have high capital costs, high operating costs, large spatial footprints, and large carbon footprints. Microbial fuel cells, such as the MFC described herein, could be installed to treat effluent from a number of processes that generate or discharge an effluent with a sufficient BOD, such as waste water treatment plants. The bacteria consumes nutrients from the water and produce power. The use of microbial fuel cells is a very clean and efficient method of energy production.
Since the power generated from a microbial fuel cell is directly proportional to the nutrient content of the source. The nutrient content of source water may be evaluated as biochemical oxygen demand (“BOD”) values. A microbial fuel cell BOD sensor can be used to measure real time BOD values. An additional preferable benefit of a microbial fuel cell is the reduction of BOD from the influent to the effluent. Such measurements may indicate the efficacy of a cell.
In a microbial fuel cell, oxygen and nitrate are preferred electron acceptors and act through reduction of oxygen and nitrate to generate current. Most microorganisms have an outer cellular structure which shows strong non-conductivity. The typical microbial fuel cells the cathode module and the anode module are separated from each other with a membrane and current generation occurs in the anode module and a cation-exchange membrane is used to transfer protons from the anode module to the cathode module. In other microbial fuel cells a barrier is used and limitations have been found in the distance from the anode to the cathode.
The present inventors have also found that naturally occurring microorganisms found in source water streams, such as nutrient rich effluent streams (i.e., with sufficient BOD levels), can directly transfer electrons previously generated from oxidation of organic substances in an anode module and can be naturally cultured during the operation of a biofuel cell.
In previously developed microbial fuel cells, where the cathode module and anode module have been separated from each other, the generation and transfer of electrons and protons by means of bio-reaction in an anode module and the consumption of electrons and protons occurs by means of the following reaction:4e+4H++O2−→2H2OTypically, a circuit is formed across a membrane for operating microbial fuel cells for source water treatment in a continuous manner.
Thus, in the typical microbial fuel cell, a cation-exchange membrane has been used to transfer protons from the anode module to the cathode module. If microorganisms in an anode module are sufficiently cultured during the process, electrons and protons are generated from the oxidation of water nutrients, such as glucose or cellulose. Generated protons may be transferred via a cation-exchange membrane while electrons are transferred through an external electric coupling. In the cathode module the electrons and protons are consumed in a reduction process involving oxygen or nitrogen. In combination, the oxidation process in the anode and the reduction process in the cathode creates a potential difference and a current may be drawn through an external electrical coupling. Cation-exchange membranes, however, are subject to fouling, which reduces the efficiency of cation transfer across a cation-exchange membrane. Moreover, because the anode module is a closed chamber, fouling can cause a pH decrease in the anode module which could detrimentally affect the micro-organisms or require the use of a buffer solution. In addition to fouling problems, membranes are very expensive and introduce cost issues, particularly at higher throughputs.
As an alternative to the use of a membrane, US Published App. No. 20050208343 teaches using glass wool or glass beads as a barrier between the cathode and the anode. However, this negatively impacts the scalability of the microbial fuel cell and fouling will still impact performance of the microbial fuel cell. The published application also limits the distance between the anode and cathode, likely due to the use of such barriers. Such distance issues are overcome in the embodiments described herein which overcome these limitations and disadvantages, and the impact of these problems with a barrier free microbial fuel cell that uses flow to prevent backflow of oxygen to the cathode chamber.